Method of inhibiting the corrosiveness of aqueous solutions of sodium bisulfate towards stainless steels



tats I Michael'A. Streicher, Wilmington, DeL, assignor to E. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware No Drawing. Application April 15, 1954 Serial No. 423,506 a 3 Claims. (Cl. 23-121) This invention relates to the corrosion inhibition of an acid salt as regards stainless typesteels, and particularly to a method of inhibiting the corrosiveness in aqueous solution of sodium bisulfate by the incorporation of ferric ion.

Sodium bisulfate is used in large quantities, either alone or in conjunction with organic acids, such as oxalic acid, for example, as a cleaner for stainless steels, the steel being Washed with an aqueous solution of the salt, or salt and acid mixture, and then the metal rinsed with fresh water. With the advent of stainless steel-sheathed locomotives and railway passenger cars and the growing use of stainless steel fixtures for restaurants, stores and in other merchandising applications, the maintenance of the original bright finish of these alloys has become of widespread concern. Sodium bisulfate-containing cleaning compositions are effective in removing grime and other foreign matter from stainless steels but are definitely corrosive to these steels and, furthermore, often cause a black discoloration which is objectionable for esthetic and other reasons. Stainless steel equipment is also utilized in chemical manufacture, and it is desirable to inhibit the corrosiveness of sodium bisulfate-containing solutions toward these steels quite part from esthetic considerations.

A primary object of this invention is to provide a method of inhibiting the corrosiveness of sodium bisulfate in aqueous solution toward stainless steels. Another object of this invention is to provide a method of inhibiting the corrosiveness of sodium bisulfate in aqueous solution While preventing objectionable contamination of the solutions, and while retaining the original bright surface of the stainless steel intact. Yet another object of this invention is to provide a method of inhibiting the corrosiveness in aqueous solution of sodium bisulfate, with or without one or more of the acids hereinafter mentioned added, for stainless steels which is economical and not dangerous to using personnel.

Generally, the objects of this invention are obtained by incorporating in the salt solution a minor amount of ferric ion by addition to the solution of a ferric salt, preferably a salt of substantial solubility, after which the solution is employed in its customary manner without regard to the fact that the inhibitor is present. The inhibited solution may also contain wetting and dispersing agents to facilitate cleaning, where this is the objective, or may contain oxalic acid or one or more of the group consisting of formic, acetic, sulfamic, and glycolic acids either singly or in mixture, the inhibitory effect of ferric ion as regards oxalic acid being taught in my Patent 2,793,190 and as regards formic, acetic, sulfamic, and glycolic acids in my Patent 2,793,191, both filed on the same date as the instant application.

The following table is a compilation of data covering a number of tests which demonstrate the efiicacy of the method of this invention as regards sodium bisulfate alone in boiling aqueous solution, the ferric ion being intro- .duced by addition of a ferric salt to the solutions, the

2,853,364 Patented Sept. 23, 1958 Uninhib- Minimum ited Oor- Amount of rosion Rate, "Fe Giving g./sq. Complete dm./l1r. Inhibition, g./liter v NaHSOr Concentration in Percent by Weight Steel, AISI Designation Unless Otherwise Indicated It will be understood that A. I. 5.1; 304 steel is the I conventional stainless type containing 18% Cr-8% Ni,

Whereas A. I. S. I. 430 and A. I. S. I. 446 are both free of Ni but contain 17% Cr and 25% Cr, respectively.

The steel analyzing 16% Cr17% Mnl% Ni is representative of the so-called substitute stainless steels, which have recently been utilized extensively in an effort to conserve nickel for national defense purposes, and this invention is also applicable to alloys of this class for which reason the term stainless steels, as used in the appended claims, is intended to comprehend both the conventional and the substitute stainless steels.

It should be mentioned that the quantities of iron reported for attainment of complete inhibition as regards type 304 steel have proved, within 120%, equally effective as regards the A. I. S. I. 300 series steels as a class, including, specifically, 301, 302, 316 and other types.

The tests reported in the table were conducted over periods of 7 hours using boiling sodium bisulfate solutions in the concentrations reported, the concentration level being maintained constant throughout by the use of condensers which returned all material boiled off tothe metal sample-containing flask during the course of the tests. Additional tests, not reported in the table, confirmed the corrosion inhibitory action of ferric ion for much greater periods of time, up to and beyond 60 hours and, independently, for exposures of the steels at lower temperatures. In all of the tests the original bright finish of the steels was preserved intact upon exposure to the corrosion-inhibited salt solutions.

ion into the salt solutions for inhibition purposes, not

only because corrosive halide residues might remain on the metal surfaces, if not removed by thorough rinsing, but also because I have found that the halogen anion exerts an inhibition-counteracting effect on the ferric ion above certain limiting total ferric salt concentration levels. In all cases investigated the ferric salt concentration at which the inhibition-counteracting effect occurred was found to be considerably in excess of the ferric ion concentration which it was necessary to maintain for complete corrosion inhibition, and therefore halogen salts are effective for the purposes of this invention, with the limitations mentioned.

All of the data hereinbefore set out is convertible to the basis of penetration in inches per month by simply multiplying by the factor 0.36.

The mechanism responsible for the corrosion inhibition of sodium bisulfatecontaining aqueous solutions toward stainless steels, while keeping the surface bright, is not understood; however, the effect appears to result from a specific property of the ferric ion itself.

From the foregoing it will be understood that my invention comprises an effective method of inhibiting the corrosiveness of sodium bisulfate in aqueous solution quantity of a ferric ion-contributing salt, but not less than an amount yielding fromabout 0.12 to about 0.65

gram/ liter of said solution of ferric iron, depending uponthe concentration of sodium bisulfate in said solution and the steel involved, and thereafter contacting said stainless steels with said solution.

2. The method according to claim 1 in which said ferric ion-contributing salt is free of halogen.

3. The method of, substantially inhibiting the corrosion of sodium bisulfate in aqueous solution toward stainless steels comprising incorporating in said solution a minor quantity of a ferric ion-contributing salt furnishing a minimum ranging fromabout 0.12 to about 0.65 gram/ liter of said solution of ferric iron, depending upon the concentration of sodium bisulfate in the solution and the steel involved, but less than the corrosion inhibitioncounteracting equivalent of the anion of said salt, and thereafter contacting said stainless steels with said solution.

References Cited in the file of this patent UNITED STATES PATENTS 1,599,996 Fritz Sept. 14, 1926 2,238,651 Keenen Apr. 25, 1941 2,666,209 Sherva Jan. 19, 1954 2,761,765 Matthews Sept. 1956 FOREIGN PATENTS 632,533 Great Britain Nov. 28, 1949 

1. THE METHOD OF SUBSTANTIALLY INHIBITING THE CORROSION OF SODIUM BISULFATE IN AQUEOUS SOLUTION TOWARD STAINLESS STEELS COMPRISING INCORPORATING IN SAID SOLUTION A MINOR QUANTITY OF A FERRIC ION-CONTRIBUTNG SALT, BUT NOT LESS THAN AN AMOUNT YIELDING FROM ABOUT 0.12 TO ABOUT 0.65 GRAM/LITER FO SAID SOLUTION OF FERRIC IRON, DEPENDING UPON THE CONCENTRATION OF SODIUM BISULFATE IN SAID SOLUTION AND THE STEEL INVOLVED, AND THEREAFTER CONTACTING SAID STAINLESS STEELS WITH SOLUTION. 